1. Field of the Invention
The present invention relates to a filter for very short electromagnetic waves and comprises a plurality of resonators which are coupled to one another and which are operated in the dual mode, whose first and last resonators, as viewed in the direction of energy transmission, are provided with connection lines for the supply and discharge of electromagnetic energy, and more particularly to such a filter in which an additional coupling is provided between at least two filter circuits which are not directly consecutive in the electrical mode of operation and in which the individual resonators are arranged in juxtaposed rows.
2. Description of the Prior Art
In the microwave technique it is well known to construct filters from a plurality of microwave resonators which are coupled to one another, the coupling being effected either capacitively or inductively. The resonators themselves can comprise, for example, so-called coaxial line resonators or wave guide resonators.
In contrast to filters constructed with concentrated switching elements, because of the geometrically predetermined configuration of the resonators, it is not possible to readily transfer every circuit which can be constructed in the concentrated technique to the microwave frequency range. This difficulty occurs, in particular, when it is necssary to produce attenuation poles in the attenuation characteristic of the filter, and/or to effect a leveling of transit time in the pass band of the filter by means of additional couplings of filter circuits. This difficulty is eliminated by the arrangement disclosed in the German published application No. 1,942,867, of the resonators in neighboring rows having additional over-couplings in the common partition wall of two resonators which are arranged in different rows.
The possibility of constructing microwave filters with cavity resonators which are simultaneously operated in more than one mode is also known. For example, one may take reference to the article entitled "Microwave Filters Employing a Single Cavity Excited in More Than One Mode", published in the "Journal of Applied Physics", Vol. 22, No. 8, August 1951, by Wei-Gaun Lin, and to the article by A. E. Williams entitled "A Four Cavity Elliptic Waveguide Filter", published in the "IEEE Transactions on Microwave Theory Techniques", Vol. MTT. 18, No. 12, December 1970. Here, preferably two identical, but orthogonal modes, are employed in H.sub.101 resonators or H.sub.111 resonators, and are coupled to one another by a coupling screw arranged at 45.degree. to the direction of the E vectors (dual mode). In this manner, two electric oscillating circuits of a filter can be constructed in a single cavity resonator in a technically effective manner. Because of the saving in respect of weight and volume up to 50% which can be achieved in dual mode operation, an important sphere of application resides, in particular, in satellite technology, particularly as the filters employed therein are subject to high electric requirements which is manifest in a relatively large number of electric oscillating circuits.
Since these filters also require attenuation poles and/or a leveling of the transit time in the pass band, it is highly desirable to provide suitable filter circuits in the dual mode technique. In this context, a proposed realization is already known from the article of A. E. Atia and A. E. Williams entitled "Nonminimum-Phase Optimum-Amplitude Bandpass Waveguide Filters", published in the "IEEE Transactions on Microwave Theory and Techniques", Vol. MTT-22, No. 4, April 1974, which, however, is restricted to filter circuits which are symmetrical, both in respect of structure and in respect of element values, and which furthermore possess frequently overlapping, additional couplings whose number and geometric position within the filter arrangement cannot be preselected, and the number of electric oscillating circuits of which must amount to a multiple of four, as a result of which this proposal frequently cannot be put into practice. A prerequisite of the construction of filters with additional couplings in the dual mode technique is, in fact, a resonator arrangement which permits a correct-sign realization of all couplings.
A possibility of improving the ability to realize filter circuits which are asymmetrical, in particular in respect of element values, and which are operable in the dual mode has been disclosed in the German published application No. 2,511,800 and resides in arranging the resonators in neighboring rows, and providing a different number of resonators in the rows.